B-fast is the industry leader in regard to ball diff parts. I was turned onto them by a fellow racer and have never gone to any stock parts since. The diff rings are ready to use out of the package with not only a mega flat surface, but cross hatching which helps retain diff lube and assure long life. They offer both carbide and ceramic parts, and allow you to build your own kits to suit your needs.
My personal favorite is the Pro Driver diff kits, my receipt is Carbide Balls, Ceramic Thrust.